Or rather, not-writing.
Not-writing can be an important part of one's writing life. We obsess about not-writing. "I'm not writing." "I have writer's block." "I didn't write this week, I'm going to hell."
Side note: I think a lot of what people call writer's block isn't, in the strictest sense. See Elizabeth Moon's essay Help! I can't write!.
But back to me, me, me.
I think not-writing is necessary. Sure, writing every day is good. Very, very good. But sometimes one needs an emotional recharge. I use up a lot on a story. After finishing a first draft, I often think and feel nothing except profound relief that the sumbitch is outta my face. Sometimes I rush right into another project. Sometimes, I just chill. I read, or watch accumulated DVDs. I [gasp!] socialize.
If writing was the only thing keeping food on my table, I might not have the luxury of not-writing. This is one advantage of writing-as-avocation.
I have another idea: writing isn't always putting things onto paper or into a computer. I sincerely believe that a lot of my writing takes place outside of my conscious mind. I call it my backbrain. "My backbrain is chewing on that scene." Then six months later my backbrain, uh, regurgitates, and I can write the scene. And often it comes out good. (Not like actual regurgitate at all!)
Sometimes when I'm not-writing, I'm walking around, and scenes are playing out in my head. This hasn't happened lately, but I think that's due more to personal events in this year, and that it will come back. In my mind, I will go over the scene several times, making subtle changes each time. Sometimes what I actually write down is different from the mental tv version, but the mental tv has made a difference, and saved me some actual writing time.
And not-writing allows time for business: filing away contracts, critiquing for my workshop, reading other books in my genre/s, going to cons, thinking about writing. Thinking about not-writing.